Color conversion apparatus, color conversion method, and recording medium

ABSTRACT

Device color signals are converted such that a total amount of used color materials in the first image area, in which image granularity is more important, is not reduced from a total amount before the device color signals are converted, and/or such that a total amount of used color materials in the second image area, in which cost of the used color materials is more important, is not increased from a total amount before the device color signals are converted.

CROSS-REFERENCE TO RELATED APPLICATIONS AND PRIORITY CLAIMS

This application is a Continuation of International Application No.PCT/JP2012/050623 filed on Jan. 13, 2012, which was published under PCTArticle 21(2) in Japanese, which is based upon and claims the benefit ofpriority from Japanese Patent Application No. 2011-044261 filed on Mar.1, 2011, the contents all of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a color conversion apparatus, a colorconversion method, and a recording medium for converting colors of aprint produced using a plurality of color materials.

BACKGROUND ART

With significant advances in inkjet technology in recent years, it hasbecome possible for inkjet image forming devices to produce large colorprints of high quality at high speeds. Such inkjet image forming devicesare widely used particularly in sign and display applications, and areapplicable, for example, to prints on POP (Point Of Purchase) posters,wall posters, outdoor advertisements, billboards, etc. Such inkjet imageforming devices are capable of producing prints by forming a number ofink dots of several tens μm in diameter on a print medium by applyingdroplets of inks in different colors, e.g., C, M, Y, K inks, to theprint medium.

It is known in the art that, according to human visual responsecharacteristics, human color vision is most sensitive to shades of gray.More specifically, if dots printed in different colors are of the samesize, then dots in a K colored ink can be perceived better visually thandots in other C, M, Y colored inks. Therefore, the observer of an imageproduced using an increased amount of K colored ink sees the image asgrainy in its entirety (granularity is high), although the observercannot identify the individual dots.

If dots printed in C, M, Y colored inks are placed closely together toform gray areas (composite black areas), then such dots provide asmoothing effect due to being superimposed on each other, therebylowering image granularity. However, since three inks are used ratherthan one ink, the total amount of inks used increases, resulting in ahigh running cost (hereinafter referred to simply as an increased“cost”).

According to inkjet technology, therefore, there is a trade-off betweengranularity and cost, and it is important to design images so thatgranularity and cost of the images is well-balanced. Various colorconversion techniques have been proposed in the art for appropriatelycontrolling the total amount of inks used by finely adjusting the dotrecording ratio while keeping the dots substantially isochromatic.

Japanese Laid-Open Patent Publication No. 2009-241609 discloses anapparatus for and a method of judging whether chromatic or achromaticdots are to be formed based on an input signal. The publication statesthat a pale K ink is used instead of a dark K ink in a color area wheredot shapes are visible, thereby increasing granularity.

Japanese Laid-Open Patent Publication No. 10-044475 discloses anapparatus for and a method of correcting the amount of each recorded inkin order to increase the density of a certain ink (a Y ink among C, M,Y, K inks) the granularity of which is least visible.

SUMMARY OF INVENTION

Input signals, based on which prints are produced, are available inseveral different attributes and types. One image area includes regionsin which many more noticeable colors are present, and other regions inwhich many less noticeable colors are present. One image area alsoincludes regions in which many low spatial frequency components arepresent, and other regions in which many high spatial frequencycomponents are present.

According to the apparatuses and methods disclosed in Japanese Laid-OpenPatent Publication No. 2009-241609 and Japanese Laid-Open PatentPublication No. 10-044475, combinations of color materials simply arechanged depending on whether or not they belong to a certain color area.In certain input images, granularity and cost may not be well balanced,but only one of granularity or cost is improved depending on the imagecharacteristics.

The present invention has been made in order to solve the aboveproblems. An object of the present invention is to provide a colorconversion apparatus, a color conversion method, and a recording medium,which are capable of producing prints that are both low in granularityand low in cost.

According to the present invention, there is provided a color conversionapparatus for converting colors of a print that is produced using aplurality of color materials, comprising a color signal input unit forinputting device color signals associated with respective amounts of thecolor materials, an image area extractor for extracting a first imagearea in which image granularity is more important and/or a second imagearea in which cost of used color materials is more important, from amongan image area represented by the device color signals input by the colorsignal input unit, and an isochromatic converter for converting thedevice color signals representing the image area into new device colorsignals, so that the new device color signals will fall within anisochromatic range in a device-independent color space, wherein theisochromatic converter converts the device color signals such that atotal amount of used color materials in the first image area extractedby the image area extractor is not reduced from a total amount beforethe device color signals are converted, and/or such that a total amountof the used color materials in the second image area extracted by theimage area extractor is not increased from a total amount before thedevice color signals are converted.

As described above, the image area extractor extracts a first image areain which image granularity is more important and/or a second image areain which the cost of used color materials is more important, from amongan image area represented by the device color signals, and theisochromatic converter converts the device color signals such that thetotal amount of used color materials in the first image area extractedby the image area extractor is not reduced from the total amount beforethe device color signals are converted, and/or such that the totalamount of used color materials in the second image area extracted by theimage area extractor is not increased from the total amount before thedevice color signals are converted. Accordingly, it is possible toproduce a print that is both low in granularity and low in cost byselectively increasing or reducing the total amount of used colormaterials depending on the image area.

The image area extractor preferably extracts the first image area and/orthe second image area, based on a contour of a given object detected inthe image area. The isochromatic converter preferably designates one ofa plurality of pixels within the image area as a pixel of interest, andconverts the device color signals depending on whether or not the pixelof interest falls within the first image area and/or the second imagearea. The isochromatic converter preferably converts the device colorsignals such that the total amount of the used color materials in thefirst image area is increased from the total amount before the devicecolor signals are converted.

If a total amount of used color materials in another image area apartfrom the first image area is to be reduced, the isochromatic converterpreferably converts the device color signals in order to prevent thetotal amount of the used color materials in the first image area frombeing reduced.

The color materials preferably include an achromatic color material anda plurality of chromatic color materials, and if the chromatic colormaterials can be combined to reproduce a color on the print producibleby the achromatic color material, the isochromatic converter preferablyconverts the device color signals such that an amount of used achromaticcolor material in the first image area is reduced from an amount beforethe device color signals are converted, and a total amount of usedchromatic color materials in the first image area is increased from atotal amount before the device color signals are converted. Thegranularity in the first image area can thus be improved by increasingthe total amount of color materials used to produce a print.

The color conversion apparatus preferably further comprises anisochromatic range determiner for determining the isochromatic range,and the isochromatic converter preferably converts the device colorsignals so as to fall within the isochromatic range determined by theisochromatic range determiner, and so as to minimize the amount of usedachromatic color material. Accordingly, granularity can be optimized inthe isochromatic range.

The isochromatic converter preferably converts the device color signalssuch that the total amount of the used color materials in the secondimage area is reduced from the total amount before the device colorsignals are converted.

If the total amount of used color materials in another image area apartfrom the second image area is to be increased, the isochromaticconverter preferably converts the device color signals in order toprevent the total amount of the used color materials in the second imagearea from being increased.

The color materials preferably include an achromatic color material anda plurality of chromatic color materials, and if the chromatic colormaterials can be combined to reproduce a color on the print producibleby the achromatic color material, the isochromatic converter preferablyconverts the device color signals such that an amount of used achromaticcolor material in the second image area is increased from an amountbefore the device color signals are converted, and a total amount ofused chromatic color materials in the second image area is reduced froma total amount before the device color signals are converted.Accordingly, the cost to produce the second image area can selectivelybe lowered by reducing the total amount of color materials used toproduce a print.

The color conversion apparatus preferably further comprises anisochromatic range determiner for determining the isochromatic range,wherein the isochromatic converter preferably converts the device colorsignals so as to fall within the isochromatic range determined by theisochromatic range determiner, and so as to maximize the amount of theused achromatic color material. Thus, the cost of color materials usedin the isochromatic range can be optimized.

The image area extractor preferably extracts, as the second image area,an image area that remains after the first image area is excluded fromthe image area. Thus, the total amount of used color materials can bereduced as much as possible, excluding the first image area in whichimage granularity is more important.

The image area extractor preferably extracts a facial area as the firstimage area.

The image area extractor preferably extracts, as the first image area,an achromatic flat area in which saturation is smaller than a firstthreshold value, and in which spatial frequency is lower than a secondthreshold value.

According to the present invention, there also is provided a colorconversion method for converting colors of a print that is producedusing a plurality of color materials, comprising using a computer toperform the steps of inputting device color signals associated withrespective amounts of the color materials, extracting a first image areain which image granularity is more important and/or a second image areain which cost of used color materials is more important, from among animage area represented by the device color signals that have been input,and converting the device color signals representing the image area intonew device color signals, so that the new device color signals will fallwithin an isochromatic range in a device-independent color space,wherein the step of converting comprises the step of converting thedevice color signals such that a total amount of used color materials inthe extracted first image area is not reduced from a total amount beforethe device color signals are converted, and/or such that a total amountof the used color materials in the extracted second image area is notincreased from a total amount before the device color signals areconverted.

According to the present invention, there further is provided anon-transitory recording medium storing a program for converting colorsof a print that is produced using a plurality of color materials, theprogram enabling a computer to function as a color signal input unit forinputting device color signals associated with respective amounts of thecolor materials, an image area extractor for extracting a first imagearea in which image granularity is more important and/or a second imagearea in which cost of used color materials is more important, from amongan image area represented by the device color signals input by the colorsignal input unit, and an isochromatic converter for converting thedevice color signals representing the image area into new device colorsignals, so that the new device color signals will fall within anisochromatic range in a device-independent color space, wherein theisochromatic converter converts the device color signals such that atotal amount of used color materials in the first image area extractedby the image area extractor is not reduced from a total amount beforethe device color signals are converted, and/or such that a total amountof the used color materials in the second image area extracted by theimage area extractor is not increased from a total amount before thedevice color signals are converted.

With the color conversion apparatus, the color conversion method, andthe recording medium according to the present invention, a first imagearea in which image granularity is more important and/or a second imagearea in which the cost of used color materials is more important isextracted from an image area represented by the device color signals,and the device color signals are converted such that the total amount ofused color materials in the first image area is not reduced from thetotal amount before the device color signals are converted, and/or suchthat the total amount of used color materials in the second image areais not increased from the total amount before the device color signalsare converted. Accordingly, it is possible to produce a print that isboth low in granularity and low in cost, by selectively increasing orreducing the total amount of used color materials depending on the imagearea.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of a printing system, which incorporatestherein a color conversion apparatus according to an embodiment of thepresent invention;

FIG. 2 is an electric block diagram of the color conversion apparatusshown in FIG. 1;

FIG. 3 is a flowchart of an operation sequence implemented by the colorconversion apparatus;

FIG. 4A is a view showing a visual image, which is represented by devicecolor signals;

FIG. 4B is a view showing first image areas, which are extracted by animage area extractor shown in FIG. 2;

FIG. 5A is a graph showing the relationship between types and amounts ofused inks prior to performing a conversion process;

FIG. 5B is a graph showing the relationship between types and amounts ofused inks after a GCR conversion process is performed;

FIG. 5C is a graph showing the relationship between types and amounts ofused inks after an IGCR conversion process is performed;

FIG. 6 is a diagram showing an isochromatic range within adevice-independent color space;

FIG. 7A is a graph showing conversion characteristics of an ordinarycolor conversion table for reproducing shades of gray;

FIG. 7B is a graph showing conversion characteristics of a GCR colorconversion table for reproducing shades of gray;

FIG. 7C is a graph showing conversion characteristics of an IGCR colorconversion table for reproducing shades of gray;

FIG. 8 is a flowchart of an operation sequence implemented by a colorconversion apparatus according to a modification of the presentinvention.

DESCRIPTION OF EMBODIMENTS

A color conversion method according to a preferred embodiment of thepresent invention, in relation to a color conversion apparatus and aprinting system that carry out the color conversion method, will bedescribed in detail below with reference to the accompanying drawings.In the descriptions that follow, forming an image may also be referredto simply as “printing”.

FIG. 1 shows in block form a printing system 10, which incorporatestherein a color conversion apparatus according to an embodiment of thepresent invention.

As shown in FIG. 1, the printing system 10 basically includes a colorconversion apparatus 12 (computer), an image forming apparatus 14, a DTP(DeskTop Publishing) apparatus 16, and a database server 18. The colorconversion apparatus 12, the DTP apparatus 16, and the database server18 are connected electrically to each other through a wired or wirelesslink.

The color conversion apparatus 12 converts input image data (devicecolor signals or page description data) supplied from an externalapparatus into device color signals, which are suitable for use in theimage forming apparatus 14 to produce a print based thereon. The colorconversion apparatus 12 outputs the converted device color signals tothe image forming apparatus 14. The device color signals represent imagedata defined as device-dependent data, e.g., raster-format data, such asTIFF data, bitmap data, RAW data, or the like, and having color channelsin four colors of C, M, Y, K or three colors of R, G, B. Thedevice-dependent data supplied to the image forming apparatus 14 mayalso have a unique data format with a desired header added thereto.

The image forming apparatus 14 is electrically connected to the colorconversion apparatus 12 through a serial interface such as a USB(Universal Serial Bus) cable, an IEEE1394 cable, an Ethernet (registeredtrademark) cable, a wireless network, or the like, or a parallelinterface such as a Centronics cable.

The image forming apparatus 14 is an inkjet printer for forming an imageon a medium (recording medium), not shown, by discharging ink dropletsfrom a recording head assembly 20 while the medium is fed in apredetermined direction. The medium has a base, which comprises a papermedium such as synthetic paper, thick paper, aluminum-evaporated paper,or the like, a resin medium such as vinyl chloride, PET, or the like, ortarpaulin paper, or the like.

The recording head assembly 20 comprises four line heads 24 c, 24 m, 24y, 24 k for discharging droplets of four color materials of differentcolors, comprising a C ink 22 c (chromatic color material), an M ink 22m (chromatic color material), a Y ink 22 y (chromatic color material),and a K ink 22 k (achromatic color material). The C ink 22 c, the M ink22 m, the Y ink 22 y, and the K ink 22 k may hereinafter be referred tocollectively as “inks 22”.

Each of the line heads 24 c, 24 m, 24 y, 24 k has a plurality ofnozzles, not shown, arrayed along the widthwise direction of the medium.The C ink 22 c, the M ink 22 m, the Y ink 22 y, and the K ink 22 k arestored in respective ink tanks 26 c, 26 m, 26 y, 26 k. The line head 24c discharges the C ink 22 c supplied from the ink tank 26 c through thenozzles thereof. The line head 24 m discharges the M ink 22 m suppliedfrom the ink tank 26 m through the nozzles thereof. The line head 24 ydischarges the Y ink 22 y supplied from the ink tank 26 y through thenozzles thereof. The line head 24 k discharges the K ink 22 k suppliedfrom the ink tank 26 k through the nozzles thereof.

The recording head assembly 20 may have an ink droplet propellingmechanism of any of various different types. For example, the recordinghead assembly 20 may have an actuator in the form of a piezoelectricdevice as an ink droplet propelling mechanism, which propels and ejectsdroplets of inks 22 upon mechanical deformation thereof based on acontrol signal. Alternatively, the recording head assembly 20 may have athermal jet mechanism as an ink droplet propelling mechanism, whichpropels and ejects droplets of inks 22 under the pressure of air bubblesthat are generated upon the inks 22 being heated by a heater. Therecording head assembly 20 is not limited to a line head assembly, butmay be a multipass head assembly for reciprocally scanning the mediumtransversely to form an image on the medium;

The DTP apparatus 16 is capable of editing material data made up ofcharacters, figures, pictures, photos, etc. The DTP apparatus 16generates electronic manuscripts in a page description language(hereinafter referred to as “PDL”) by laying out the material data oneach page. PDL refers to a language which is descriptive of imageinformation including format information, position information, colorinformation (including density information), etc., of characters,figures, etc., in a “page” that serves as an output unit for printing,displaying, or the like. The DTP apparatus 16 performs a rasterizingprocess on electronic manuscripts described in PDL format. Therasterizing process includes a data format converting process forconverting PDL format data into raster format data, and a colorconverting process using a target profile.

The database server 18 is an apparatus for managing data such as jobtickets of electronic manuscripts, e.g., JDF (Job Definition Format)files, color sample data, target profiles, or device profiles suitablefor a combination of the image forming apparatus 14 and the medium.

FIG. 2 is an electric block diagram of the color conversion apparatus 12shown in FIG. 1.

The color conversion apparatus 12 includes an input interface 30 (colorsignal input unit), a controller 32, a memory 34 (recording medium), andan output interface 36. The memory 34 stores a program for controllingthe controller 32 to function as the color conversion apparatusaccording to the present embodiment.

The input interface 30 receives electric signals from externalapparatus. For example, the input interface 30 acquires device colorsignals and PDL data, which are edited and generated by the DTPapparatus 16, and also acquires various items of information such as ICC(International Color Consortium) profiles or the like stored in thedatabase server 18.

The output interface 36 sends electric signals to external apparatus.For example, the output interface 36 supplies device color signals tothe image forming apparatus 14, which have been processed by a colorconversion process according to the present invention.

The color conversion process is a process for converting only the signalvalue of a device color signal without changing the types of colorplates (e.g., C, M, Y, K signal data). A device color signal prior toconversion thereof will hereinafter be referred to as a “pre-conversioncolor signal”, and a device color signal after conversion thereof willbe referred to as a “post-conversion color signal”.

The input and output device color signals (pre-conversion color signalsand post-conversion color signals) are associated with respectiveamounts of used inks 22 in the image forming apparatus 14. Such anassociation may be desirable for each color channel. For example, theused amount “0%” may be assigned to a device color signal representing alowest gradation level, and the used amount “100%” may be assigned to adevice color signal representing a highest gradation level. Linearlychanging used amounts may be assigned to device color signalsrepresenting intermediate gradation levels between the lowest andhighest gradation levels.

The controller 32, which comprises an information processor such as aCPU or the like, includes a rasterizing processor 38, an analytic imagedata generator 40, an image area extractor 42, an isochromatic converter44, and an isochromatic range determiner 46.

The rasterizing processor 38 carries out the same rasterizing functionas the DTP apparatus 16. The rasterizing processor 38 performs arasterizing process on input data depending on the type of input data.If device color signals are directly supplied to the input interface 30,then the rasterizing processor 38 does not need to perform a rasterizingprocess on the input data.

The analytic image data generator 40 generates analytic image data froma pre-conversion color signal, which is acquired from the inputinterface 30 or the rasterizing processor 38. More specifically, theanalytic image data generator 40 performs a color conversion process forconverting the pre-conversion color signal into device-independent data.The term device-independent data refers to data defined in a colorsystem, such as HSV (Hue-Saturation-Value), HLS(Hue-Lightness-Saturation), CIELAB, CIELUV, XYZ, or the like.

The analytic image data generator 40 generates L*a*b* data, YCC data, orthe like from the pre-conversion color signal using print profilesdepending on the image forming apparatus 14, for example.

The image area extractor 42 includes a granularity-valued area extractor48 for extracting from a given image area a first image area in whichimage granularity is more important, and a cost-valued area extractor 50for extracting from the given image area a second image area in whichthe cost of used inks 22 is more important.

In the first image area, a color conversion process is carried out so asnot to reduce the total amount of used inks 22. In other words, thefirst image area refers to an image area, which is specified to improvedot-induced granularity or to prevent granularity from increasing, byincreasing or maintaining the total amount of used inks 22.

In the second image area, a color conversion process is carried out soas not to increase the total amount of used inks 22. In other words, thesecond image area refers to an image area, which is specified to reducethe cost of used inks 22 or to prevent the cost of used inks 22 fromincreasing, by reducing or maintaining the total amount of used inks 22.

The isochromatic converter 44 includes a pixel-of-interest designator 52for designating one of a plurality of pixels in an image arearepresented by device color signals, an image area attribute determiner54 for determining an image area attribute of a given pixel (a pixel ofinterest), and a color signal converter 56 for converting device colorsignals into new device color signals.

The color signal converter 56 includes an IGCR (Inverse Gray-ComponentReplacement) converter 58 for carrying out an IGCR conversion process,and a GCR (Gray-Component Replacement) converter 60 for carrying out aGCR conversion process. The IGCR conversion process and the GCRconversion process are effective in the present embodiment (see theimage forming apparatus 14 shown in FIG. 1) in cases where the C ink 22c, the M ink 22 m, and the Y ink 22 y, which are chromatic colormaterials, can be combined to reproduce a color on the print 28producible by the K ink 22 k, which is an achromatic color material.Details of the IGCR conversion process and the GCR conversion processwill be described later.

The memory 34 stores print profile data 62 suitable for the imageforming apparatus 14, an IGCR conversion table 64 used in the IGCRconversion process, and a GCR conversion table 66 used in the GCRconversion process. The memory 34 may additionally store pre-conversioncolor signals, post-conversion color signals, analytic image data, andvarious items of information required for the color conversion processaccording to the present invention.

The color conversion apparatus 12 according to the present embodiment isbasically configured as described above. Operations of the colorconversion apparatus 12 will be described in detail below with referenceto the flowchart shown in FIG. 3.

First, device color signals are input to the color conversion apparatus12 through the input interface 30 (step S1). The color conversionapparatus 12 is supplied with device color signals in two signal entrymodes.

According to the first signal entry mode, the DTP apparatus 16 generatesa PDL electronic manuscript according to an editing process, and thenrasterizes the generated PDL electronic manuscript into device colorsignals, i.e., C, M, Y, K signal data, for use in printing. Thegenerated device color signals are supplied from the DTP apparatus 16 tothe color conversion apparatus 12 through the input interface 30.

According to the second signal entry mode, the DTP apparatus 16generates a PDL electronic manuscript according to an editing process,and then supplies the generated PDL electronic manuscript to the imageforming apparatus 14. Thereafter, the rasterizing processor 38 reads theprint profile data 62 stored in the memory 34, and rasterizes thesupplied PDL electronic manuscript into device color signals, i.e., C,M, Y, K signal data, for use in printing.

Thereafter, the analytic image data generator 40 performs a given colorconversion process on the device color signal in order to generateanalytic image data (step S2). The analytic image data generator 40generates, for example, YCC data for use in a later-described facedetecting process, and L*a*b* data for use in a later-describedisochromatic conversion process.

The supplied device color signals and the generated analytic image dataare temporarily stored in the memory 34.

Thereafter, the image area extractor 42 extracts a granularity-valuedarea in which image granularity is more important from an image arearepresented by the device color signals (step S3). More specifically,the granularity-valued area extractor 48 reads the device color signalsand the analytic image data from the memory 34, and then applies variousimage detecting algorithms to the device color signals and the analyticimage data to extract a granularity-valued area. Specific examples ofthe image detecting algorithms will be described below.

In the first example, a detected facial area may be set as agranularity-valued area, due to the fact that the human face is atypical object, the image quality of which can easily be evaluated asgood or bad.

A facial area may be detected by any of various known image processingtechniques disclosed in Japanese Laid-Open Patent Publication No.2006-285959 and Japanese Laid-Open Patent Publication No. 2007-148537.The analytic image data that are used to detect the facial area may beany of various types. For example, if YCC data are used, then a knowndetecting process incorporated in an input device such as a digitalcamera or the like may be applied as is.

In the second example, an achromatic flat area may be set as agranularity-valued area, due to the fact that human visual responsecharacteristics (contrast resolution) are maximum for gray colors and aspatial frequency band in the vicinity of 1.0 Cy/mm. An achromatic flatarea refers to an image area in which saturation is smaller than a firstthreshold value and in which the spatial frequency is lower than asecond threshold value. More specifically, the first threshold value maybe 5 and the second threshold value may be 1.0 Cy/mm (the number ofcorresponding pixels varies depending on resolution). The first andsecond threshold values are not limited to the above values, and may beset to desired values depending on printing conditions and the manner inwhich the print will be observed.

In the third example, a flat image area having a memory color may be setas a granularity-valued area. The term memory color refers to a colorwhich is recalled and stored in association with something by a humanobserver. For example, the memory color associated with the human faceis a flesh tone color, the memory color associated with a sunset glow isorange, and the memory color associated with cherry flowers is pink. Aflat image area may be detected not only in view of the color of theimage area, but also in view of other attributes of the scene.

In the second and third examples, the flat area may be detectedaccording to any of known pattern detecting algorithms including aFourier transform, a wavelet transform, a pattern matching algorithm,and a static process, etc. In addition, the flat area may be detected asa cluster of pixels by performing a joined component labeling process onthe analytic image data or the device color signals.

In this manner, a granularity-valued area is extracted from an imagearea 100 shown in FIG. 4A. FIG. 4A is a view showing a visual image,which is represented by device color signals. The image area 100represents a natural picture of the upper half of the body of anupstanding woman positioned substantially in the center of the imagearea 100.

FIG. 4B is a view showing first image areas 102 and 104, which areextracted from the image area 100 by the image area extractor 42. Thefirst image area 102, which is located substantially centrally in theimage area 100, corresponds to a detected facial area. The first imagearea 104, which is located in a lower right region of the image area100, corresponds to a detected achromatic flat area.

The cost-valued area extractor 50 extracts a cost-valued area from theimage area 100, in which the cost of used inks 22 is more important. Thecost-valued area is a second image area 106, which is illustrated as ablank area in FIG. 4B. The second image area 106 is an image area thatremains after the first image areas 102, 104 have been excluded from theimage area 100. Therefore, the total amount of used inks 22 in the imagearea 100 can be minimized, except for the first image areas 102, 104 inwhich image granularity is more important. The image area extractor 42supplies area attribute information (a granularity-valued area or acost-value area) for each pixel of the image area 100 to the memory 34,which temporarily stores the supplied area attribute information.

In this manner, the image area extractor 42 extracts agranularity-valued area from the image area 100 (step S3). In steps S4through S8, color signal values of the device color signals areconverted successively for each pixel.

The pixel-of-interest designator 52 designates one of a plurality ofpixels within the image area 100 as a pixel of interest (step S4). Forexample, the pixel-of-interest designator 52 may designate a pixelaccording to an ascending order of addresses assigned to respectivepixels of the image area 100. However, the pixel-of-interest designator52 may designate a pixel according to any order.

Next, the image area attribute determiner 54 determines whether or notthe present pixel of interest falls within a granularity-valued area(step S5). More specifically, the image area attribute determiner 54 maydetermine whether the present pixel of interest belongs to agranularity-valued area or a cost-valued area, by referring toinformation concerning the present pixel of interest from among the areaattribute information read from the memory 34.

Thereafter, depending on the result determined by the image areaattribute determiner 54, the color signal converter 56 selects a processfor converting the color signal value of the pixel of interest. If theimage area attribute determiner 54 determines that the present pixel ofinterest belongs to a granularity-valued area, the color signalconverter 56 uses the IGCR conversion process (step S6). On the otherhand, if the image area attribute determiner 54 determines that thepresent pixel of interest does not belong to a granularity-valued area,the color signal converter 56 uses the GCR conversion process (step S7).

Details of the IGCR conversion process and the GCR conversion processwill be described below.

FIG. 5A is a graph showing a relationship between types of inks andamounts of used inks prior to performing a conversion process. FIG. 5Bis a graph showing the relationship between types of inks and amounts ofused inks after the GCR conversion process is performed. FIG. 5C is agraph showing the relationship between types of inks and amounts of usedinks after the IGCR conversion process is performed.

In FIG. 5B, the amounts of the C ink 22 c, the M ink 22 m, and the Y ink22 y are reduced respectively by ΔC, ΔM, ΔY from the amounts of the Cink 22 c, the M ink 22 m, and the Y ink 22 y shown in FIG. 4A. In orderto cancel out a density reduction caused by such a reduction in theamounts of the inks 22, the K ink 22 k is increased by ΔK. In thismanner, the total amount of used inks 22 is reduced by (ΔC+ΔM+ΔY−ΔK)while the color reproducibility of the print 28 (see FIG. 1) is keptsubstantially constant.

In FIG. 5C, the amounts of the C ink 22 c, the M ink 22 m, and the Y ink22 y are increased respectively by ΔC, ΔM, ΔY from the amounts of the Cink 22 c, the M ink 22 m, and the Y ink 22 y shown in FIG. 4A. In orderto cancel out a density increase caused by such an increase in theamounts of the inks 22, the K ink 22 k is reduced by ΔK. In this manner,the total amount of used inks 22 is increased by (ΔC+ΔM+ΔY−ΔK) while thecolor reproducibility of the print 28 (see FIG. 1) is kept substantiallyconstant. Since the coverage ratio of the formed dots becomes higher,granularity is improved as a whole.

FIG. 6 is a diagram showing an isochromatic range in adevice-independent color space. In the present embodiment, the inks 22are available in four types, i.e., C, M, Y, K. In FIG. 6, forillustrative purposes, the inks are available in three types, i.e., C,M, Y.

It is assumed that a point, which corresponds to a device color signalvalue prior to a conversion process, is denoted by P. In adevice-independent color space, e.g., a L*a*b* color space, a rangewithin which the color difference Δe with a color value corresponding tothe point P is 0.5 or smaller is referred to as an isochromatic range R.In the color conversion process, any desired device color signal withinthe isochromatic range R is handled as being isochromatic with respectto the point P.

As shown in FIG. 6, a color that falls within the isochromatic range Raround the point P and at which the K value (%) is maximum isrepresented by a point Q1. The GCR conversion table 66 is generated bydetermining points Q1 for each of respective points P in adevice-dependent color space. A color that falls within the isochromaticrange R around the point P and at which the K value (%) is minimum isrepresented by a point Q2. The IGCR conversion table 64 is generated bydetermining points Q2 for each of respective points P in adevice-dependent color space.

The point Q1 and the point Q2 may be determined by any of various searchalgorithms. For example, using the print profile data 62 (see FIG. 2),L*a*b* color values may be calculated with respect to all colors in thevicinity of the point P, and then an optimum CMYK combination, whichsatisfies certain limiting conditions, may be determined. The optimumCMYK combination may be determined not only in view of colorapproximations, but also in view of gradation characteristics (e.g.,continuity and smoothness of a gradation curve).

Characteristics of the GCR conversion table 66 and the IGCR conversiontable 64, which are obtained in the foregoing manner, will be describedbelow. FIGS. 7A through 7C are graphs showing conversion characteristicsof color conversion tables for reproducing shades of gray. Each of thegraphs has a horizontal axis representing pre-conversion color signalvalues (%) and a vertical axis representing post-conversion color signalvalues (%).

The graph illustrated in FIG. 7A shows conversion characteristics of anordinary color conversion table. The graph does not represent anequivalent transformation (Y=X), but rather represents a linear curve,the gradient of which is much smaller than 1. This is because the totalamount of used inks 22 is limited to less than 400%. The respectiveamounts of used inks 22 are the same at each gradation level of gray.

The graph illustrated in FIG. 7B shows conversion characteristics of theGCR conversion table 66. The K value represents an equivalenttransformation (Y=X) in a range of 0≦K≦Th1, and represents a curve witha high gradient in a range of K≧Th1, which is saturated in the vicinityof 100% in a range of high color signal values. Another color (e.g., theC value) represents an equivalent transformation (Y=X) in a range of0≦C≦Th1, and represents a curve with a low gradient in a range of K≧Th1,which increases monotonically up to a maximum range (100%). In a rangebeyond Th1, the ratio of K is greater than the ratio of other colors (C,M, Y). As can be seen from FIG. 7B, the GCR conversion table 66 iseffective to reduce the total amount of used inks 22 in a range from Th1to 100%.

The graph illustrated in FIG. 7C shows conversion characteristics of theIGCR conversion table 64. The K value represents 0% at all times in arange of 0≦K≦Th2, and is non-zero and sharply increases in a range ofK≧Th2. Another color (e.g., the C value) represents an upwardly convexmonotonically increasing function, which reaches a maximum value of 100%at C=Th2, and then gradually decreases in a range of K≧Th2. Within thisrange, the ratio of K is smaller than the ratio of other colors (C, M,Y). As can be understood from FIG. 7C, the IGCR conversion table 64 iseffective to increase the total amount of used inks 22 in a range from0% to 100%.

As shown in FIG. 3, the color signal value of the pixel of interest isconverted according to the IGCR conversion process of the IGCR converter58 (step S6), or according to the GCR conversion process of the GCRconverter 60 (step S7), using the IGCR conversion table 64 or the GCRconversion table 66, which have the above conversion characteristics.

The memory 34 may store a plurality of IGCR conversion tables 64 and aplurality of GCR conversion tables 66 depending on the size (e.g., anallowable color difference Δe) of the isochromatic range R. If thememory 34 stores a plurality of IGCR conversion tables 64 and aplurality of GCR conversion tables 66, then the isochromatic rangedeterminer 46 can select a given combination of an IGCR conversion table64 and a GCR conversion table 66 by determining in advance the size ofthe isochromatic range R, in view of the accuracy of color reproduction.

Then, the pixel-of-interest designator 52 determines whether or not theconversion process has been completed on all of the pixels of the imagearea 100 (step S8). If the pixel-of-interest designator 52 determinesthat the conversion process has not been completed on all of the pixels,then the pixel-of-interest designator 52 designates another one of thepixels that has not yet been presently designated (step S4). Theisochromatic converter 44 then repeats steps S4 through S8 successively.

If the pixel-of-interest designator 52 determines that the conversionprocess has been completed on all of the pixels, then the colorconversion apparatus 12 outputs converted device color signals (C, M, Y,K signals) through the output interface 36 to the image formingapparatus 14 (step S9).

The image forming apparatus 14 converts the supplied device colorsignals into control signals, which are used in controlling discharge ofinks 22 from the recording head assembly 20. The recording head assembly20 discharges inks 22 based on the supplied control signals. Thedischarged droplets of inks 22 are applied to the surface of anon-illustrated medium, thereby forming a number of dots on the mediumto print an image (print 28). On the print 28, granularity caused by thedots is improved or is prevented from increasing in the first imageareas 102, 104, and the cost of used inks 22 is lowered or is preventedfrom increasing in the second image area 106.

As described above, the first image areas 102, 104, in which imagegranularity is more important, and the second image area 106, in whichthe cost of used inks 22 is more important, are extracted from the imagearea 100, and device color signals are converted such that the totalamount of used inks 22 in the first image areas 102, 104 will not bereduced from the total amount before the conversion process, and/or suchthat the total amount of used inks 22 in the second image area 106 willnot be increased from the total amount before the conversion process.Accordingly, it is possible to produce a print 28 that is both low ingranularity and low in cost by selectively increasing or reducing thetotal amount of used inks 22 depending on the image area 100.

The isochromatic converter 44 may convert device color signals in orderto increase the total amount of used inks 22 in the first image areas102, 104 from the total amount before the conversion process. If thetotal amount of used inks 22 in another image area (the second imagearea 106 in FIG. 4B) apart from the first image areas 102, 104 isreduced, then the isochromatic converter 44 may convert device colorsignals in order to prevent the total amount of used inks 22 in thefirst image areas 102, 104 from being reduced. In this manner, the sameadvantages as those described above can also be achieved.

The isochromatic converter 44 may also convert device color signals inorder to reduce the total amount of used inks 22 in the second imagearea 106 from the total amount used before the conversion process. Ifthe total amount of used inks 22 in other image areas (the first imageareas 102, 104 in FIG. 4B) apart from the second image area 106 isintended to be reduced, then the isochromatic converter 44 may convertdevice color signals in order to prevent the total amount of used inks22 in the second image area 106 from being increased. In this manner,the same advantages as those described above can also be achieved.

A modification of the present embodiment will be described below withreference to the flowchart shown in FIG. 8. Steps S11, S12, and S20 ofFIG. 8 are identical to steps S1, S2, and S9 of FIG. 3 and hence willnot be described below.

In step S13, the image area extractor 42 extracts a granularity-valuedarea and a cost-valued area from an image area 100 represented by thedevice color signals. In the modified embodiment, the second image area106 (see FIG. 4), which is a remaining image area, is set as acost-valued area. Similar to the case of the first image areas 102, 104,the second image area 106 may be extracted according to any of variousimage detecting algorithms. For example, an image area, the granularityof which is difficult to be recognized visually, may be set as acost-valued area. More specifically, such an image area may be an imagearea represented by chromatic colors of relatively low brightnessincluding many high spatial-frequency components.

Similar to the above-described embodiment, color signal values of thedevice color signals are converted successively for each pixel in stepsS14 through S19. However, three color conversion processes (steps S16through S18) are carried out according to the modification, whereas twocolor conversion processes (steps S6 and S7) are carried out accordingto the above-described embodiment.

In step S15, the image area attribute determiner 54 determines theattribute of the present pixel of interest. If the image area attributedeterminer 54 determines that the present pixel of interest belongs tothe granularity-valued area, the color signal converter 56 uses the IGCRconversion process (step S16). If the image area attribute determiner 54determines that the present pixel of interest belongs to the cost-valuedarea, then the color signal converter 56 uses the GCR conversion process(step S17). If the image area attribute determiner 54 judges that thepresent pixel of interest belongs to another image area, the colorsignal converter 56 uses an equivalent transformation (step S18). Inother words, a color conversion process is not carried out on an imagearea that does not belong to the granularity-valued area or thecost-valued area.

Therefore, images can be designed strictly depending on the image area100 by selectively increasing, reducing, or maintaining the total amountof used inks 22. Therefore, prints that are both low in granularity andlow in cost can easily be produced.

Alternatively, a plurality of levels for ascribing values to thegranularity of images may be established stepwise, and color conversionprocesses may be selected depending on such levels. The same approachmay also be employed with respect to the cost of the used inks 22.

The present invention is not limited to the above embodiment. Variouschanges and modifications may be made to the embodiment withoutdeparting from the scope of the invention.

For example, in the above embodiment, a color conversion process iscarried out on each pixel within the image area 100 after agranularity-valued area or a cost-valued area has been extracted.However, a color conversion process may be carried out simultaneouslywith extraction of a granularity-valued area or a cost-valued area. Adesignated pixel of interest may be detected as described above andjudged as belonging to a granularity-valued area or not, and thereafter,a color conversion process may be performed on the designated pixel ofinterest.

In the above embodiment, the four color plates in colors of C, M, Y, Khave mainly be described. However, the principles of the presentinvention are also applicable to desired types of color plates and adesired number of color plates. For example, standard inks in colors ofC, M, Y, K may be combined with optional inks in pale colors such as LC,LM, or the like, and W (white).

The image forming apparatus 14 is not limited to an inkjet printer, butmay be any type of printer capable of producing dots by applying colormaterials to a print medium.

1. A color conversion apparatus for converting colors of a print that isproduced using a plurality of color materials, comprising: a colorsignal input unit for inputting device color signals associated withrespective amounts of the color materials; an image area extractor forextracting a first image area in which image granularity is moreimportant and/or a second image area in which cost of used colormaterials is more important, from among an image area represented by thedevice color signals input by the color signal input unit; and anisochromatic converter for converting the device color signalsrepresenting the image area into new device color signals, so that thenew device color signals will fall within an isochromatic range in adevice-independent color space, wherein the isochromatic converterconverts the device color signals such that a total amount of used colormaterials in the first image area extracted by the image area extractoris not reduced from a total amount before the device color signals areconverted, and/or such that a total amount of the used color materialsin the second image area extracted by the image area extractor is notincreased from a total amount before the device color signals areconverted.
 2. The color conversion apparatus according to claim 1,wherein the image area extractor extracts the first image area and/orthe second image area, based on a contour of a given object detected inthe image area.
 3. The color conversion apparatus according to claim 1,wherein the isochromatic converter designates one of a plurality ofpixels within the image area as a pixel of interest, and converts thedevice color signals depending on whether or not the pixel of interestfalls within the first image area and/or the second image area.
 4. Thecolor conversion apparatus according to claim 1, wherein theisochromatic converter converts the device color signals such that thetotal amount of the used color materials in the first image area isincreased from the total amount before the device color signals areconverted.
 5. The color conversion apparatus according to claim 1,wherein, if a total amount of used color materials in another image areaapart from the first image area is to be reduced, the isochromaticconverter converts the device color signals in order to prevent thetotal amount of the used color materials in the first image area frombeing reduced.
 6. The color conversion apparatus according to claim 4,wherein the color materials include an achromatic color material and aplurality of chromatic color materials, and if the chromatic colormaterials can be combined to reproduce a color on the print producibleby the achromatic color material, the isochromatic converter convertsthe device color signals such that an amount of used achromatic colormaterial in the first image area is reduced from an amount before thedevice color signals are converted, and a total amount of used chromaticcolor materials in the first image area is increased from a total amountbefore the device color signals are converted.
 7. The color conversionapparatus according to claim 6, further comprising: an isochromaticrange determiner for determining the isochromatic range, wherein theisochromatic converter converts the device color signals so as to fallwithin the isochromatic range determined by the isochromatic rangedeterminer, and so as to minimize the amount of the used achromaticcolor material.
 8. The color conversion apparatus according to claim 4,wherein the isochromatic converter converts the device color signalssuch that the total amount of the used color materials in the secondimage area is reduced from the total amount before the device colorsignals are converted.
 9. The color conversion apparatus according toclaim 4, wherein, if the total amount of used color materials in anotherimage area apart from the second image area is to be increased, theisochromatic converter converts the device color signals in order toprevent the total amount of the used color materials in the second imagearea from being increased.
 10. The color conversion apparatus accordingto claim 8, wherein the color materials include an achromatic colormaterial and a plurality of chromatic color materials, and if thechromatic color materials can be combined to reproduce a color on theprint producible by the achromatic color material, the isochromaticconverter converts the device color signals such that an amount of usedachromatic color material in the second image area is increased from anamount before the device color signals are converted, and a total amountof used chromatic color materials in the second image area is reducedfrom a total amount before the device color signals are converted. 11.The color conversion apparatus according to claim 10, furthercomprising: an isochromatic range determiner for determining theisochromatic range, wherein the isochromatic converter converts thedevice color signals so as to fall within the isochromatic rangedetermined by the isochromatic range determiner, and so as to maximizethe amount of the used achromatic color material.
 12. The colorconversion apparatus according to claim 1, wherein the image areaextractor extracts, as the second image area, an image area that remainsafter the first image area is excluded from the image area.
 13. Thecolor conversion apparatus according to claim 1, wherein the image areaextractor extracts a facial area as the first image area.
 14. The colorconversion apparatus according to claim 1, wherein the image areaextractor extracts, as the first image area, an achromatic flat area inwhich saturation is smaller than a first threshold value, and in whichspatial frequency is lower than a second threshold value.
 15. A colorconversion method for converting colors of a print that is producedusing a plurality of color materials, comprising using a computer toperform the steps of: inputting device color signals associated withrespective amounts of the color materials; extracting a first image areain which image granularity is more important and/or a second image areain which cost of used color materials is more important, from among animage area represented by the device color signals that have been input;and converting the device color signals representing the image area intonew device color signals, so that the new device color signals will fallwithin an isochromatic range in a device-independent color space,wherein the step of converting comprises the step of converting thedevice color signals such that a total amount of used color materials inthe extracted first image area is not reduced from a total amount beforethe device color signals are converted, and/or such that a total amountof the used color materials in the extracted second image area is notincreased from a total amount before the device color signals areconverted.
 16. A non-transitory recording medium storing a program forconverting colors of a print that is produced using a plurality of colormaterials, the program enabling a computer to function as: a colorsignal input unit for inputting device color signals associated withrespective amounts of the color materials; an image area extractor forextracting a first image area in which image granularity is moreimportant and/or a second image area in which cost of used colormaterials is more important, from among an image area represented by thedevice color signals input by the color signal input unit; and anisochromatic converter for converting the device color signalsrepresenting the image area into new device color signals, so that thenew device color signals will fall within an isochromatic range in adevice-independent color space, wherein the isochromatic converterconverts the device color signals such that a total amount of used colormaterials in the first image area extracted by the image area extractoris not reduced from a total amount before the device color signals areconverted, and/or such that a total amount of the used color materialsin the second image area extracted by the image area extractor is notincreased from a total amount before the device color signals areconverted.